Few cases of HIV transmission between women who have sex with women (WSW) have been reported in the United States, but these cases still can and do occur. Studies have shown that HIV can be transmitted by female-to-female sexual contact with unprotected exposure to vaginal or other bodily fluids and to blood from menstruation. Historically, confirming HIV transmission during female-to-female sexual contact has been difficult because other risk factors are almost always present or cannot be ruled out. “It can be difficult to determine if HIV was transmitted by female-to-female sex or other more common modes of transmission, such as injection drug use and heterosexual sex,” says Amy Lansky, PhD, MPH. A Case Report According to a report published in the Morbidity & Mortality Weekly Report, the Houston Depart­ment of Health contacted the CDC in August 2012 regarding a rare transmission of HIV that likely resulted from sexual contact between HIV-discordant partners. The women involved reported having unprotected sex during a 6-month monog­amous relationship. The woman with newly acquired HIV did not report any other recognized risk factors for HIV infection. The other partner was previously diagnosed with HIV and had stopped receiving antiretroviral treatment in 2010. In this case, laboratory tests confirmed that the woman with newly diagnosed HIV had a virus that was virtually identical to that of her partner. “This case was unique because the CDC was able to use both phylogenetic and epidemiologic data in the investigation,” Dr. Lansky says. The viruses infecting the two women had a 98% or higher sequence identity in three genes. The couple had not received any preventive counseling before...